The Caspian Sea (Azerbaijani: Xəzər dənizi, Persian: دریای خزر, Russian: Каспийское море, Kazakh: Каспий теңізі, Turkmen: Hazar deňizi) is the largest enclosed body of water on Earth by area, variously classed as the world's largest lake or a full-fledged sea. The sea has a surface area of 371,000 km² (143,200 sq mi) (not including Garabogazköl Aylagy) and a volume of 78,200 km³ (18,800 cu mi). It is in an endorheic basin (it has no outflows) and is bounded to the northwest by Russia, to the west by Azerbaijan, to the south by Iran, to the southeast by Turkmenistan, and to the northeast by Kazakhstan.

The ancient inhabitants of its littoral perceived the Caspian Sea as an ocean, probably because of its saltiness and seeming boundlessness. It has a salinity of approximately 1.2%, about a third the salinity of most seawater. The Caspian Sea has been called Gilan (دریای گیلان) on ancient maps. In Iran, it is referred to as Daryâ-ye Mazandaran (دریای مازندران), meaning "the Sea of Mazandaran" in Persian, and sometimes also as Daryâ-ye Khazar.